Thursday, December 25, 2008

It's snowing. A lot. This morning, there was even a little snow on the walk under the eaves, outside my front door. And some tiny dinosaurs had been hopping there, looking for seeds in the dead plants:

Can I count this toward my list of geological things I've done?

(Also, since ice is a mineral, is snow a sediment or a volcanoclastic deposit? I know it's not metamorphic until it's been buried in a glacier, or at least buried in the snowpack and recrystallizes.)

10 comments:

I have argued since the eighties that in addition to the traditional groups of sedimentary, igneous and metamorphic, we need to have at least one more minor group, sublimates- minerals that solidify directly from a gas. Fumarolic deposits, vapor phase deposition in volcanics, and snow would fall into this category. I honestly do think that as we continue our exploration of astronomical bodies, this will come to be recognized as an important class.

The Ediacaran Blog yesterday had a wonderful post about why dinosaurs hate Christmas, and I think my family is getting tired of hearing about it. My two younger nephews are going to be irritated when I start yelling "Dinosaurs! Outside! Looking for food!"

Andrew- the problem is if you define chemical sedimentary rock as broadly as you have- "species that come out of solution from fluids-" then all igneous rocks fall into chemical sedimentary as well. The definition that I learned was "minerals that precipitate from aqueous solution." That definition would exclude igneous rocks, but it would also exclude minerals that crystallized directly from a gaseous state.

Melts and solutions are really the same thing, especially if you're treating them thermodynamically. John Brady (mineralogy/petrology prof at Smith College) gives a great partial melting/fractional crystallization lecture using things like salty water (especially to demonstrate eutectic melting) and popsicles. I started sitting up and thinking when he commented that, really, the solution of salt into a salt/water liquid was the same process as a silicate mineral dissolving in a silicate melt.

So I think that all rocks that crystallize from solution, including vein minerals (which are usually precipitates from a high-temperature aqueous solution), rock salt, gypstone, etc., should be considered igneous.

When we talked igneous petrology, we generally referred to the magma as a "silicate solution." One of my great regrets was that I never took physical chemistry, (and it just seems too dense for me to figure out by myself) but I think Kim is right in that thermodynamically a "solution" is a "melt" of some material, whether it's water or silicate. I think I mentioned in passing in my piece for "Geology in Spaaaace!" that we have never really learned to think of water as a lithic material, though I think that will change as we continue to explore colder solid bodies in our solar system. We tend to think of water as intrinsically liquid- which of course it isn't, any more than silica is intrinsically solid. Our special location and environment make these states "normal" from our perspective.

The 300 K provision would be very problematic- that's only 27 C, or about 80 F, so evaporites from Death Valley would be considered igneous? Furthermore, hydrothermal deposits (quartz veins and so on) would require special consideration. Actually, the latter are the other group that I think deserves it's own minor classification- depending on my mood, I can argue based on the traditional definitions that they are igneous, metamorphic, or chemical sedimentary. My own feeling is that they are odd enough and distinctive enough that they don't really fit any of the three big groups.

Kim's solution, to think of all of these things as igneous, really cuts through the Gordian Knot, but the three major groups also carry genetic (i.e. environmental) connotations, which would be lost with her approach.

I think Kim is right in that thermodynamically a "solution" is a "melt" of some material, whether it's water or silicate

It's the other way around - melts are one kind of solution. (That's the perspective from taking, ummm, four different thermodynamics courses. Don't ask me why I did that. I should have taken rock mechanics instead.)

The intro textbook that I'm currently using classifies hydrothermal veins as a fourth type of rock.

The problem is that really there's not much fundamental about the definitions of the three major rock types. I know one igneous petrologist who snarkily declared that granites are really metamorphic rocks (because the slow cooling allows them to change after they've crystallized, and they aren't as reliable geochemically as volcanic rocks). And low-grade metamorphic rocks are frequently named as if they are sedimentary rocks, especially if they still contain fossils or have a mappable stratigraphy. And then there are migmatites - they're part igneous and part metamorphic. That's one reason why asking students to start with "is it igneous, sedimentary, or metamorphic" when they're looking at rocks-in-a-box is really unfair, and why I have so much trouble answering the questions from my brother-in-law who teaches elementary school.

"It's the other way around - melts are one kind of solution." Yes, of course. I was thinking of lquids, but there are solid solutions and gaseous solutions. Poorly considered statement.

And regarding your comments on education, this has been a pet peeve of mine for decades. The focus is on nomenclature, whether it's rocks, minerals, landforms, or whatever. Geology is about interpretation, and that is the fun and exciting aspect of the discipline. We teach K-12 students the names of all sorts of things, but spend almost no time showing them how to make sense of the things they (supposedly) now the names of. Most K-12 science education is bad about this issue, but I think geology is the worst.

This is interesting! Now I've always had trouble with the "rock cycle" because it doesn't correspond to reality, it's just a teaching tool. The idea that igneous, sedimentary and metamorphic are concepts that eventually impede rather than promote further understanding is a new one for me. I do know that the boundaries between them are arbitrary, but they are still "strong attractor" concepts that describe a lot of truth.

melts and fluids aren't the same thing, because when they equilibrate with ambient surface conditions fluids are still fluid and melts aren't. That's why geysers are volcanoes in the outer solar system, but not here.

Hanging on by my nails...

About Me

I'm a forty-something tenured geology professor at a small public college in the Rockies. I love mountains - hiking in them, looking at them, studying them.
You can reach me at shearsensibility at gmail dot com.
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